The present invention relates to a pneumatic tire, more particularly to a studless tire of which tread rubber comprises short fibers to improve on-the-ice performance.
Studless tires are, in order to improve on-the-snow performance and on-the-ice performance, conventionally provided with blocks and sipes in the tread portion and soft tread rubber is used. In recent years, inventions on tread rubber which includes fibrous substances functioning as spikes have been made.
In Japanese patent application No.11-212129 filed on Jul. 27, 1999, a studless tire of which tread rubber comprises non-metallic short fibers such as glass fibers or carbon fibers oriented in the thickness direction of the tread rubber was proposed, wherein the short fibers have an average length of 0.1 to 5 mm and an average diameter of 1 to 100 micrometers. And the ratio E1/E2 of the tread rubber, which is defined as the ratio of the complex modulus E1 in the thickness direction to the complex modulus E2 in the tire circumferential direction, is set in the range of from 1.1 to 4 under a temperature of 25 deg.C., frequency of 10 Hz, initial strain of 10%, and dynamic strain of 1%. Further, at a temperature of xe2x88x9210 deg.C., the tread rubber has a hardness of from 45 to 75. Therefore, due to the adhesive friction, cohesive friction and scratching friction, the frictional force (grip) between the tire tread surface and ice-covered road surface can be effectively improved, and the wear resistance of the tread is also improved.
We handed over duties to develop this invention studied in order to further improve on-the-ice performance especially that of passenger car tires, and it was discovered that such a tread rubber displays its full power when combined with a specific tire profile and/or tread pattern. Further, it was discovered that when the tire is vulcanized in a mold as usual, the short fibers"" orientation is liable to disturbed in the neighborhood of the ground-contacting surface, and the short fibers are, against expectation, oriented in random directions which are almost parallel with the ground-contacting surface, which therefore, nullify the effects of the short fibers.
It is therefore, an object of the present invention to provide a studless tire which is based on Japanese patent application No.11-212129 but further improved in on-the-ice performance.
According to one aspect of the present invention, a studless tire comprises a tread rubber defining a ground-contacting region, the tread rubber comprising at least a diene rubber and non-metallic short fibers oriented in the tire radial direction, wherein a tire profile satisfies the following equation
1 greater than TW/S greater than 0.92xe2x88x920.17xc3x97A
wherein
TW is a ground-contacting width of the tire,
S is a section width of the tire, and
A is an aspect ratio of the tire.
In the ground-contacting region, at least one circumferential rib of which total axial width is 15 to 30% of the ground-contacting width may be disposed. Preferably, the ground-contacting face is provided on at least 80% of its area with unevenness which is molded by a tire mold to have a ten-point mean roughness of from 30 to 500 micrometers.